Solvent blockers and multilayer barrier coatings for thin films

ABSTRACT

A film comprising alternating layers of polyvinylidene chloride resin and a solvent barrier such as polyvinyl alcohol provides superior barrier properties. Barrier and mechanical properties are enhanced when formulation additives are present at customary levels only in the outermost layers.

BACKGROUND OF THE INVENTION

This invention relates to films with thick or multiple layers ofpolymeric coating and processes for preparing such films.

Several methods are known for applying thick or multiple coating layersto films or other objects. For example, U.S. Pat. No. 3,959,526discloses a process of preparing a high barrier, heat sealable packagingmaterial having a low level of total retained solvents. A sub-coating ofa high barrier vinylidene chloride copolymer and a top coating of a heatsealable vinylidene chloride copolymer are applied from solution to afilm. Application of the top coating from a dilute solution containingabout 5 to 10% solids to a solids coating weight of about 0.5 to 3 g/m²provides a low level of solvent retained in the coatings after a dryingstep.

U.S. Pat. No. 4,781,978 discloses articles with a coating used forpromoting adhesion. The coating is formed from a blend comprising atleast one polymer containing carbonylamide functional groups (e.g.polyethyloxazoline) and at least one hydrophobic polymer substantiallyfree of acidic functional groups. Coated articles such as PET film canbe prepared by evaporating a solvent carrier of the polymer blend.

U.S. Pat. No. 3,950,579 discloses a method of forming relatively thickdeposits of polymeric material on a surface of small threaded articleswhich comprises forming a thin coating on the surface of a solution ofthe polymeric material, preferably draining the excess solution, thendrying the coating, applying a barrier coating such as polyvinyl alcoholto the polymeric material to protect it from attack by solvent, andapplying a second thin coating of a solution of the polymeric material.Coating is by successive immersion in baths of the barrier material orby other bulk treatment. This treatment permits the required amount ofpolymeric material to be deposited in the grooves of the article whilepreventing articles in bulk from sticking together in an agglomeratedmass. The polymeric material is a polyurethane or preferably an acrylicor methacrylic resin, and an adhesion promoting material such as apolyamide or a silicone resin is used. Such coatings provide increasedfriction between the threaded article and a mating nut.

Other references disclose film structures with varied polymer layers.U.S. Pat. No. 4,741,970 discloses a thermoplastic laminate structurewherein one of the layers is an adhesive comprising a poly-2-oxazoline.The adhesive is well suited for adhering saran to polypropylene.

U.S. Pat. No. 4,565,742 discloses a film laminate prepared by a varietyof lamination and coating processes. The film comprises a base film ofe.g. oriented polyester or nylon, a coating of polyvinylidene chloride,and a sealant layer of e.g. ethylene vinyl acetate copolymer. A secondcoating of PVDC can be applied to the base film by heat lamination orthrough an aqueous PVDC dispersion. When the base film is nylon, thefirst PVDC coating tends to prevent water in the second PVDC dispersionfrom wrinkling or curling the underlying film.

Japanese patent application 59-152929 discloses a method for treating athermoplastic polyester container comprising coating the container witha polymer from dispersion or solution. In one example a first coating ofsaponified polyvinyl acetate is applied from solution, then a vinylidenechloride polymer latex, followed by drying.

The present invention provides a means for applying multiple layers ofsolvent-borne coatings to films without contamination ofpreviously-applied layers by the solvent. The invention also providesfilms which have outstanding barrier and processing properties.

SUMMARY OF THE INVENTION

The present invention provides a process for applying multiple layers ofpolymeric coatings to a film comprising the steps of:

(a) applying a coating of solvent-borne polymer to at least one surfaceof said film;

(b) removing substantially all of the solvent from said coating;

(c) thereafter applying to the at least one coated surface of said filma polymeric barrier layer;

(d) applying to said polymeric barrier layer a subsequent coating ofsolvent-borne polymer, carried in a solvent to which said polymericbarrier layer is substantially impermeable; and

(e) removing the solvent from the subsequent coating;

whereby the solvent carrying said subsequent coating does not permeatethe underlying coating.

The invention further provides a film prepared by the above process, andin addition a coated film having good barrier and handling properties,comprising:

(a) a substrate film;

(b) at least one underlayer of vinylidene chloride polymer containingless than about 3% by weight organic formulation additives; and

(c) at least one overlayer of vinylidene chloride polymer containing atleast about 3% by weight of organic formulation additives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show one embodiment of the present invention, with coatinglayers applied to a single side or both sides of a sheet.

FIGS. 3 and 4 similarly show a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Certain barrier coatings and other coatings are commonly applied tofilms by solvent-coating processes. Solvent coating processes aregenerally well known and are illustrated by U.S. Pat. No. 4,565,742, thedisclosure of which is incorporated herein by reference. Briefly, suchcoatings can be applied by dip tank, spray, brush, roller, doctor knife,air knife, gravure, and other methods known to those skilled in the art.After application of the solvent-borne coating, the solvent is removedby drying, normally at elevated temperatures. If thick coatings aredesired, it is often difficult or slow to adequately remove solvent,since much of it must first diffuse through a layer of polymer. Even ifthe thick coating is applied in a plurality of passes, solvent from theouter layer or layers will permeate the inner layer or layers, andsimilarly will be difficult to remove. One embodiment of the presentinvention avoids these problems by the use of one or moresolvent-barrier or solvent-blocker coatings, as more fully describedbelow.

Coatings are applied to a substrate film or base film, which can be anyof a number of polymeric materials, including polyesters such as PET,nylon, cellophane, polyolefins, paper, or paperboard.

A first layer of solvent-borne coating is applied to the substrate filmby known means. This coating may be any of a variety of polymericcoatings, such as olefin homo- and copolymers, polyesters,polyethyleneimine, polyvinyl chloride, and copolymers thereof. It ispreferred that the coating is a layer of a resin such as a vinylidenechloride (VDC) polymer. VDC copolymers are copolymers of 65 to 96% byweight of vinylidene chloride and 4 to 35% of one or more comonomerssuch as vinyl chloride, acrylonitrile, methacrylonitrile, methylmethacrylate, or methyl acrylate, and are generally referred to assaran. A suitable grade contains about 7 weight percentmethacrylonitrile, 3 weight percent methyl methacrylate, and 0.3 weightpercent itaconic acid comonomers. Many VDC polymers customarily includeformulation additives such as waxes, fatty acids and derivativesthereof, particulate matter, and polymeric species such as mixedpolyesters. Such additives may be useful to reduce brittleness andstiffness of the polymer or to improve its heat sealability and handlingproperties.

The first polymeric layer, if it is a VDC polymer composition, isnormally applied from a 10-30 weight percent solution in a solventmixture of tetrahydrofuran and toluene in a ratio of about 50:50 toabout 80:20 (alternatively, methyl ethyl ketone and toluene) by knowntechniques, followed by drying in a hot air dryer. The thickness of sucha layer, in order to exhibit acceptable drying time, characteristicallycorresponds to about 1 to about 4 and preferably about 2 to about 3 gsolids/m² surface area. The present invention provides a means forobtaining an effectively greater thickness of such a layer by use of apolymeric barrier layer.

Next in succession after the first polymeric layer is the polymericsolvent-barrier layer. This is normally a polymeric layer which isselected to be substantially impermeable to the solvent used to applythe first layer (and the top layer, which is normally, but notnecessarily, the same as the first layer). When VDC polymer is used asthe first layer, suitable polymeric barrier layers may be prepared frompolyvinyl alcohol, preferably at least 95% saponified; copolymers ofethylene and vinyl alcohol wherein the amount of copolymerized ethyleneis up to about 50 mole %, preferably up to about 30 mole %; methylmethacrylate vinyl alcohol copolymers, polyoxazoline, mixtures of thesematerials, and the like. Alternatively a layer of an inorganic glasscoating could be effective. Polyvinyl alcohol, if used, may contain asmall amount of a crosslinking agent (about 1 to about 10% by weight) inorder to improve adhesion and reduce moisture sensitivity. Thesematerials are for practical purposes insoluble in and impermeable to thetoluene-tetrahydrofuran solvent used for VDC polymer, under theconditions of time and temperature encountered in the coating processes.Such layers are applied by techniques similar to those described above,normally from an aqueous solution, typically containing about 5 to about20% dissolved polymer. Since the VDC polymer underlayer is insoluble inand not swelled by water, the application of an aqueous coating does notcontaminate the underlayer with water or other undesirable solvent.Thereafter, the polymer of the solvent-barrier layer is dried and formsan effective solvent barrier.

Atop the solvent-barrier layer is applied an outer layer of additionalpolymeric resin. This outer layer will normally be substantially similarto the inner layer, although this is not required. The outer polymer(e.g. a PVDC or acrylic formulation) is dissolved or otherwise dispersedin a solvent which does not permeate the solvent-barrier layer; thus theunder layer is not contaminated by the solvent and the film can bereadily dried. In this way a film can be prepared with a thicker overallpolymeric coating than would otherwise be economically possible.

FIG. 1 shows a schematic cross section of a film prepared by the presentinvention. Layer 11 represents the base film, which is coated with a VDCpolymer layer, 13. The solvent-barrier layer 15 is next in succession,and the outermost layer, 17, is another VDC polymer layer. While theabove process has been set forth as coating a single side of the basefilm, it is possible to coat both sides simultaneously, resulting in asymmetrical, multiple layered structure. Such a structure is shown inFIG. 2, in which layers 13', 15', and 17' correspond to layers 13, 15,and 17, but are located on the opposite side of the base film 13. Othercoating schemes are also possible. For example, the above process can berepeated as desired to build up a multiple layer structure of anyarbitrary degree of thickness.

Such structures, provided with separate and distinct VDC polymer layers,provide the opportunity for precise tailoring of each layer to perform aspecific function. For example, VDC polymer layers can be used for theirbarrier properties to prevent permeation by gases such as oxygen, watervapor, or carbon dioxide. It has now been found that VDC polymers withorganic formulation additives entirely absent or present at only a lowlevel have better barrier properties than do comparable VDC polymerscontaining formulation additives. Using the process of the presentinvention, it has been found possible to provide a film with one or moreunder layers of VDC polymer containing only small amounts of formulationadditives which thereby provides outstanding barrier performance. Theouter or outermost layer of VDC polymer can contain formulationadditives in an amount sufficient to provide heat sealability, ease ofhandling, static resistance, resistance to blocking, machinability, heatseal jaw release, good ink receptivity, and adhesion. The polyvinylalcohol, which is used as a solvent-barrier layer, can itself provideadditional barrier properties to the final structure. The resultingcomposite film structure has the unusually combination of very goodbarrier properties and good mechanical and handling properties.

In addition, use of an underlayer having only a small amount of organicformulation additive permits better wetting when an aqueous solution ofpolyvinyl alcohol is applied thereto and provides better adhesionbetween the layers in the resulting structure.

In order to exhibit excellent barrier properties, the under layer of VDpolymer should contain less than about 3 weight percent of theformulation additives. It is believed that the barrier propertiesimprove as the amount of such additives decrease, so that it ispreferred that the amount of the formulation additives be less thanabout 1 weight percent, and for best results less than about 1/2 weightpercent. Conversely, for good mechanical and handling properties, theamount of such additives in the surface layer should be at least about 3weight percent and may suitably be 6 or 7 weight percent or even as highas 10 weight percent. Either VDC polymer layer may contain small amountsof inorganic materials which do not interfere with the operation of thepresent invention. Examples of such inorganic materials include finelydivided talc, which serves as an antiblocking agent and calcium oxide,which serves as an acid scavenger.

Certain of the advantages of the present invention relating to improvedbarrier and handling properties can be obtained even when a multiplelayered film is prepared without use of a solvent-barrier layer. FIG. 3illustrates such a film. Layer 21 is a base film, as described above. Onthe base film is a layer 23 of VDC polymer containing only a very lowlevel of organic formulation additives, as described. On the surface isa layer 27 of VDC polymer containing a higher level of such additives,suitable to provide the aforementioned desired properties. FIG. 4 showsa corresponding structure in which the coating layers are applied toboth sides. Although such structures may not be dried so readily as thestructures which incorporate the solvent-barrier layer, the ultimatebarrier properties are exceptional.

Films of the present invention may be used for wrapping products thatneed oxygen, aroma, or moisture protection. Examples includerefrigerated entrees, cookies, cereals, baked goods, shelf-stable drysoups, flavored drinks, cheeses, and soaps. The barrier properties canextend product shelf-life, secure product flavor-aroma, and preventundesirable odor from getting into the package. In addition to providingexcellent barrier properties, the present invention provides benefits ofmachinability, printability, clarity, and heat sealability.

EXAMPLES 1-7 AND COMPARATIVE EXAMPLES C1-C4

Films of PET about 28 cm wide were coated using a 28 cm (11 inch) widedoctor roll coater fitted with an unwind and a windup stand for flexiblefilm. On this device film is unwound and pulled through a dip pancontaining the coating bath. The wet film is pulled through the doctoror metering rolls and into a 2 meter (7 foot) dryer where the solvent isremoved. Dried film is wound up at a windup station outside the dryer. Afirst coating of VDC polymer (about 90% vinylidene chloride, 7%methacrylonitrile, 3% methyl methacrylate, and 0.3% itaconic acidcomonomers, in a coating formulation set forth more fully in ComparativeExample C5) was applied by doctor roll coating a solution of about 15%of the polymer in a solvent mixture of 65% tetrahydrofuran and 35%toluene. This first coating was applied in a thickness indicated inTable I, the reported thicknesses being accurate to about ±10%. Thesolvent was removed and the film dried by passing the film at 23 m/min(75 feet/min) through a hot air dryer at 135°-140° C. for a residencetime of 5.6 seconds. Thereafter a barrier layer of polyvinyl alcohol wasapplied using similar equipment. The polyvinyl alcohol was a medium-lowviscosity, fully hydrolyzed resin, Elvanol® 90-50, available from DuPont. It was applied as an aqueous solution containing about 10%dissolved solids and dried by passing at 11 m/min (35 feet/min) throughan oven at 150° C. for a residence time of 12 seconds. Where noted inthe Table, the PVA layer contained a small amount of a crosslinkingagent, Cymel™ 385, which is a methylated melamine formaldehyde resinfrom American Cyanamide. In some instances an acid catalyst (phosphoricacid at levels of 0.25 to 10% of the Cymel™) was added to aidcrosslinking. Atop this barrier layer was applied a second, outercoating of VDC polymer by the same process used to apply the underlyingcoating layer of VDC polymer. The outer coating was the same compositionas the inner coating except that it contained conventional additives topromote anti-blocking, heat seal jaw release, and machinability. Theadditives amounted to about 7 percent by weight of the outer coating,including about 1 percent waxes, about 5 percent fatty acids andderivatives thereof, about 1/2 percent of a copolyester, and about 0.9percent talc. Thickness of this coating is noted in Table I.

After the final drying step, the amount of retained solvent in the filmwas measured by headspace gas chromatography. The results, in Table I,show the remarkable reduction of retained solvent, reduced by one to twoorders of magnitude compared to structures without the barrier layer(Comparative Examples C1 and C2).

The coated films of the present invention were tested for heat sealstrength by bringing the outer VDC polymer layers of two sheets of filmtogether under heat (140° C.) for 0.25 seconds at a pressure of about 34kPa (5 psig). The data in Table I show that the films of the presentinvention exhibit acceptable heat seal strength.

Oxygen transmission of certain of the films was measured using a Mocon™Ox-Trans™ instrument. The films exhibit excellent resistance to oxygenpermeation, as shown in Table I.

                                      TABLE I.sup.a                               __________________________________________________________________________        Coat 1                                                                              PVOH                                                                              Crosslink,                                                                          Coat 2                                                                              Residual   Heat Seal                                Ex  VDC g/m.sup.2                                                                       g/m.sup.2                                                                         %     VDC g/m.sup.2                                                                       THF ppm                                                                             Tol. ppm                                                                           g/cm  OTR.sup.b (%                       __________________________________________________________________________                                               RH)                                1   2.1   0.7 0     2.8   20    273  63    <.03  (0)                                                                     .03   (50)                                                                    4.2  (100)                         2   2.1   0.8 5     2.2   365   248  49    <.03  (0)                                                                     .06   (50)                                                                    4.0  (100)                         3   2.2   0.7 .sup. 5.sup.c                                                                       2.8   --    --   124   .23.sup.d                                                                          (100)                         4   2.2   0.5 .sup. 0.sup.c                                                                       1.9   55    304  85    <.03.sup.d                                                                         (100)                         5   2.0   0.6 0     2.2   79    290  67    --                                 6   2.0   0.6 .sup. 5.sup.c                                                                       2.3   57    212  83    --                                 7   2.0   0.9 .sup. 0.sup.c                                                                       2.3   34    163  75    --                                 C1  3.0   0   0     3.0   1500  2406 --    --                                 C2  2.0   0   0     2.7   2344  9016 --    --                                 C3  2.4   0   0     0     --    --   --    6.7  (100)                         C4  0     0   0     2.0   --    --   --    10.7 (100)                         __________________________________________________________________________     .sup.a Entry of a hyphen (--) indicates the measurement was not made.         .sup.b Oxygen transmission rate in cm.sup.3 /m.sup.2atm-day.                  .sup.c With acid catalyst.                                                    .sup.d Low values may be due to experimental error.                      

EXAMPLES 8 AND 9

The films of Examples 8 and 9, described in Table II, were made much asExample 1. Plant-scale coating apparatus was used, however, providing afilm width of about 1.4 m. The coating speed was 60 m/min for the firstVDC polymer layer and for the PVA layer. The first VDC polymer layer wasdried at 135°-140° C. for 23 seconds. The polyvinyl alcohol solution wasapplied by a spraying process, rather than by a doctor roll and wasdried at 150° C. for 3.4 seconds. Properties of the film are reported inTable II. Thickness of coating layers was measured at the left (1),center (c), and right (r) portions of the film.

The results show that (1) retained solvents are lower than expected forthese heavy coating weights, (2) the crosslinking agent does not appearto cause an increase in heat seal strength, and (3) high humidity OTRvalues are lower than would be expected.

                                      TABLE II                                    __________________________________________________________________________    Coat 1   PVOH                                                                              Crosslink.sup.a,                                                                    Coat 2  Residual   Heat Seal                                                                           OTR.sup.b                         Ex VDC g/m.sup.2                                                                       g/m.sup.2                                                                         %     VDC g/m.sup.2                                                                         THF ppm                                                                             Tol. ppm                                                                           g/cm  0% 50%                                                                              100%                        __________________________________________________________________________    8  (l)                                                                              2.0                                                                              0.8 4.7   4.0                      0.63                                                                             1.55                                                                             1.08                           (c)                                                                              2.0                                                                              0.8       3.5     116   210  24    0.93                                                                             3.56                                                                             1.40                           (r)                                                                              2.0                                                                              0.8       4.2                      1.24                                                                             4.03                                                                             1.08                        9  (l)                                                                              2.0                                                                              0.8 4.7   3.9                      0.46                                                                             1.86                                                                             1.55                           (c)                                                                              2.0                                                                              0.8       3.9     200   229  28    0.62                                                                             2.17                                                                             1.08                           (r)                                                                              2.0                                                                              0.8       3.3                      0.62                                                                             1.24                                                                             1.08                        __________________________________________________________________________     .sup.a Crosslinking agent is Cymel ™ 385 + 0.8% phosphoric acid, based     on dry weight of PVOH.                                                        .sup.b Oxygen transmission rate in cm.sup.3 /m.sup.2atm-day at indicated      relative humidity.                                                       

COMPARATIVE EXAMPLE C5

for reference purposes, a 12 micrometer (48 gauge) base film of PET wascoated simultaneously on both sides with a coating compositionsubstantially free from waxes and similar additives, as indicated(amounts may not add to 100% because of rounding):

    ______________________________________                                        VDC (Dow ™ F-278, as in Ex. 1)                                                                  98.0      wt. %                                          Goodyear Flexclad ™ PE-100                                                                      0.5                                                      Finely divided talc, 3-4                                                                           1.5                                                      micrometer particles                                                          Calcium oxide        0.05                                                     ______________________________________                                    

(Flexclad™ is a polyester resin having a molecular weight of at leastabout 500, prepared by condensing a glycol and a polyfunctional acid, atleast 70% of which is an aromatic acid.) This coating was applied as a15 weight percent solids solution in a solvent of 65:35 tetrahydrofuran:toluene (and about 0.1% water). The total dry coating weight was 4.2g/m². After drying and heat aging for development of VDC polymercrystallinity, oxygen transmission rate measurements were performed at25° C. and the relative humidities indicated below. Results are given incm³ /m² -day-atm.

    ______________________________________                                        % RH    Run 1         Run 2   Average                                         ______________________________________                                         0      2.22          2.11    2.17                                            50      2.29          2.25    2.27                                            80      2.57          (not determined)                                        100     2.25          2.28    2.27                                            ______________________________________                                    

COMPARATIVE EXAMPLE C6

A 12 micrometer (48 gauge) base film of PET was coated simultaneously onboth sides in the same manner as Example 5 with a VDC polymercomposition with waxes and other additives. The coating composition was:

    ______________________________________                                        VDC polymer (Dow ™ F-278)                                                                       92.7     wt. %                                           Goodyear Flexclad ™ PE-100                                                                      0.5                                                      Finely divided talc, 3-4                                                                           0.9                                                      micrometer particles                                                          Calcium oxide        0.06                                                     Behenic acid         3.1                                                      Stearamide           0.3                                                      Candelilla wax       0.8                                                      Carnauba wax         0.3                                                      Glycerol monostearate                                                                              1.5                                                      ______________________________________                                    

This coating was applied as a 15 weight percent solids solution in asolvent mixture of 65:35 tetrahydrofuran:toluene (and about 0.1% water).The total dry coating weight was 4.3 g/m². After drying and heat aging,oxygen transmission rate measurements were performed at 25° C. and therelative humidities indicated below. Results are given in cm³ /m²-day-atm.

    ______________________________________                                        % RH    Run 1         Run 2   Average                                         ______________________________________                                         0      6.23          6.74    6.49                                            50      6.11          6.42    6.27                                            80      6.14          5.81    5.97                                            100     6.77          6.15    6.46                                            ______________________________________                                    

These OTR values are signifiantly poorer than values obtained for filmwith comparable VDC polymer coating without waxes in Comparative ExampleC5.

COMPARATIVE EXAMPLE C7

A 12 micrometer (48 gauge) base film of PET was coated on one surfaceonly in the same manner as Comparative Example C5, using the VDC polymercomposition and formulation set forth in Comparative Example C6. Twowebs of this film were laminated using Adcote™ 506-40 (from Morton) with9L10 catalyst, the coated surfaces facing outward. The total coatingweight was about 4.0 g/m². OTR measured for this laminate at 25° C. and80% relative humidity was 3.9 cm³ /m² -day-atm. Although this OTR valueis better than that of Comparative Example C6 due to the doublethickness of the PET base film and the adhesive, it is still not as goodas that of the film of Comparative Example C5, which is preferablybecasue of its lower OTR and relative ease of manufacture.

EXAMPLE 10

To the film of Comparative Example C5 was added a two-sided coating ofpolyvinyl alcohol. The coating was done on a semiworks coating apparatususing a solution of 10 weight % PVOH in water. The total amount of thepolyvinyl alcohol coatings was 1.6 g/m². After drying the coatings, thefilm was additionally two-sided coated with the VDC polymer compositiondefined in Comparative Example C5. The coating weight of the final,outer layers was 4.0 g/m². No fully formulated (wax-containing) topcoating was used. After drying and heat aging, the following OTR valueswere measured at 25° C. (reported as cm³ /m² -day-atm):

    ______________________________________                                        % RH    Run 1         Run 2   Average                                         ______________________________________                                         0      0.00          0.02    0.01                                            50      0.00          0.00    0.00                                            80      1.10          1.15    1.13                                            100     1.30          1.84    1.57                                            ______________________________________                                    

The results show excellent barrier properties. The outstanding resultsat low relative humidity is attributed to the barrier properties of thepolyvinyl alcohol layer. Even at high humidities the barrier issignificantly improved, due, it is believed, to the improved propertiesof VDC polymer in the absence of waxes.

EXAMPLES 11-14

Films with multiple layers of VDC polymer coatings were prepared asdescribed generally for Example 10. In each case the first or underlayers were the VDC polymer composition described in Comparative Example5. The second or outer layers were fully formulated (wax-containing)compositions as described in Comparative Example C6. In Example 13 therewere additionally intermediate layers of polyvinyl alcohol provided(i.e., one on each side), in the amounts indicated. Details of thestructures and barrier properties of these films are shown in Table III.OTR, in cm³ /m² -day-atm, was measured at 25° C. and relative humidityas indicated; water vapor transmission rate (WVTR), in g/m² -day, wasmeasured at 38° C. and 90% relative humidity.

                                      TABLE III                                   __________________________________________________________________________    Coatings, g/m.sup.2                                                                              OTR             WVTR                                       Ex.                                                                              VDC1                                                                              PVOH                                                                              VDC2                                                                              Total                                                                             % RH                                                                              Run 1                                                                             Run 2                                                                             Run 3                                                                             Run 1                                                                             Run 2                                  __________________________________________________________________________    11 3.9 --  2.0 5.9 80  2.77                                                                              2.54                                                                              2.48                                                                              not meas'd                                 12 4.1 --  1.9 6.0 80  2.87                                                                              2.90                                                                              --  2.3 2.6                                    13 8.0 1.6 2.2 11.8                                                                              80  1.55                                                                              1.58                                                                              --  1.9 1.7                                    14 4.9 --  1.0 5.9  0  2.85                                                                              2.65                                                                              --  not meas'd                                 "                  50  1.67                                                                              1.64                                                                              --                                             "                  80  2.50                                                                              2.36                                                                              --                                             "                  100 21  22  --                                             __________________________________________________________________________

(The high OTR measurments at 100% RH are believed to be the result ofexperimental error.)

All four structures exhibit good barrier properties, the major part ofwhich appears to arise from the unformulated undercoating, VDC1. Even inthe absence of an intermediate solvent barrier layer, a good combinationof barrier properties and surface properties is obtained by limiting thepresence of waxes to the surface VDC polymer layers.

EXAMPLE 15

A multiple layer structure was prepared by combining two-sided coatingsas in Example 13, using, however, additional layers. To the base filmwas applied a first layer of the VDC composition of Comparative ExampleC5 (i.e., "VDC1"), a layer of polyvinyl alcohol, a second layer of thesame VDC composition, a second layer of polyvinyl alcohol, and finallyan outer layer of the VDC polymer composition of Comparative Example C6(i.e., "VDC2"). The results at 25° C. in cm³ /m² -day-atm are asfollows:

    __________________________________________________________________________    Coatings, g/m.sup.2        OTR                                                VDC1                                                                              PVOH VDC1                                                                              PVOH VDC2                                                                              Total                                                                              Run 1                                                                             Run 2                                                                              (% RH)                                    __________________________________________________________________________    4.2 1.6  4.0 1.2  5.0 16.0 <0.02                                                                             <0.02                                                                               (0)                                                                 <0.02                                                                             <0.02                                                                              (50)                                                                 1.04                                                                              --   (80)                                                                 1.60                                                                              1.80 (100)                                     __________________________________________________________________________

EXAMPLE 16

A film of PET 12 micrometers thick (48 gauge) was solution coated on oneside first with 0.8 g/m polyvinyl alcohol, followed by a coating of 1.0g/m² of the wax-free formulation of Comparative Example C5, followed by2.0 g/m² of the fully compounded formulation of Comparative Example C6.Drying after each coating was as described in Example 1. The film thusprepared was aged at 50° C. for at least 48 hours. Evaluation of theOxygen Transmission Rate, as described in Example 1, was performed. Theresults are as follows:

    ______________________________________                                        % RH    Run 1         Run 2   Average                                         ______________________________________                                         0      <0.06         <0.06   <0.06                                           50      0.16          0.31    0.24                                            80      6.71          6.52    6.62                                            100     18            21      --                                              ______________________________________                                    

(The high OTR at 100% RH is believed to be the result of experimentalerror.)

What is claimed is:
 1. A process for applying multiple layers ofpolymeric coatings to a film comprising the steps of:(a) applying acoating of solvent-borne polymeric composition to at least one surfaceof said film; (b) removing substantially all of the solvent from said atleast one coating; (c) thereafter applying to the coated surface of saidfilm a polymeric barrier layer; (d) applying to said polymeric barrierlayer a subsequent coating of solvent-borne polymeric composition,carried in a solvent to which said polymeric barrier layer issubstantially impermeable; and (e) removing the solvent from thesubsequent coating; whereby the solvent carrying said subsequent coatingdoes not permeate the underlying coating.
 2. The process of claim 1wherein the solvent which carries said subsequent coating of (d) is thesame solvent which carries the underlying coating of (a).
 3. The processof claim 2 wherein said polymeric barrier layer of (c) is applied from asolvent which does not permeate the underlying coating, which solvent isremoved before applying said subsequent coating.
 4. The process of claim1 wherein steps (c), (d), and (e) are repeated to provide a film withmultiple polymeric barrier layers.
 5. A process for applying multiplelayers of polymeric coatings to a film comprising the steps of:(a)applying a coating of solvent-borne polymeric composition comprising atleast one vinylidene chloride polymer to at least one surface of saidfilm; (b) removing substantially all of the solvent from said at leastone coating; (c) thereafter applying to the coated surface of said film,from a solvent comprising water, a water soluble polymeric barrierlayer; (d) applying to said polymeric barrier layer a subsequent coatingof solvent-borne polymeric composition comprising at least onevinylidene chloride polymer, carried in a solvent to which saidpolymeric barrier layer is substantially impermeable; and (e) removingthe solvent from the subsequent coating; whereby the solvent carryingsaid subsequent coating does not permeate the underlying coating.
 6. Theprocess of claim 5 wherein said polymeric barrier layer is selected fromthe group consisting of polyvinyl alcohol, ethylene vinyl alcoholcopolymers, methyl methacrylate vinyl alcohol copolymers,polyethyloxazoline, and mixtures thereof.
 7. The process of claim 6wherein said polymeric barrier layer is polyvinyl alcohol.
 8. Theprocess of claim 7 wherein the polyvinyl alcohol contains 1 to 10 weightpercent crosslinking agent.
 9. The process of claim 5 wherein saidunderlying coating of (a) and said subsequent coating of (d) comprise atleast one vinylidene chloride polymer.
 10. The process of claim 5wherein said vinylidene chloride polymer is a copolymer of at leastabout 85 weight percent polyvinylidene chloride with methacrylonitrileand methyl methacrylate.
 11. The process of claim 10 wherein the solventfrom which said coatings of vinylidene chloride polymer are applied is amixture of toluene and tetrahydrofuran.
 12. The process of claim 5wherein the solvent-borne polymer of (a) is a polymeric vinylidenechloride composition containing less than about 3 weight percent organicformulation additives and the solvent-borne polymer of (d) is apolymeric vinylidene chloride composition containing at least about 3weight percent organic formulation additives.
 13. The process of claim12 wherein the solvent-borne polymer of (a) contains less than about 1weight percent organic formulation additives.
 14. The process of claim12 wherein the solvent-borne polymer of (a) contains less than about 0.5weight percent organic formulation additives.